In photoelectric conversion devices, glass sheets with high transmittance and superior durability are used widely as the base material for the side on which light is incident. The glass sheets are used as cover glass for protecting the photoelectric conversion elements and as the substrate on which the photoelectric conversion elements are formed. In the former type of glass sheets, photoelectric conversion elements made of crystalline silicon or the like are layered on the glass sheet with a resin film made of EVA (ethylene-vinyl acetate copolymer) disposed in between, for example. In the latter type of glass sheets, photoelectric conversion elements made of thin-film silicon or the like are formed on the surface of the glass sheet with a transparent conductive film disposed in between (see for example JP H11-298030A).
A high transmittance is desired for these glass sheets. In order to increase the transmittance, coloring due to iron components in soda-lime glass may be suppressed by using a raw material with high purity. For example, JP H4-228450A discloses soda-lime glass with low iron components. This glass is a transparent glass with colored edge that contains, in mass %, less than 0.2% total iron oxide in terms of Fe2O3 as colorant, and wherein the ratio of ferrous oxide (FeO) to total iron oxide is at least 0.4. At a thickness of 5.66 mm, this glass has a light transmittance (illumination C) of at least 87%.
In order to obtain soda-lime glass having a light tint and a high transmittance while including the ordinary amount of iron oxide, it has been proposed to add an oxidizer such as cerium oxide, and to thus lower the content of FeO, which is the principal component responsible for coloring and decrease in transmittance. For example, JP H5-221683A discloses that the Fe2+/Fe3+ ratio in the glass can be lowered from the ordinary level (about 38%) to 3 to 10% by including 0.1 to 0.5 wt % of CeO2 as oxidizer in ordinary transparent soda-lime glass containing 0.06 to 0.12 wt % of iron impurities in terms of Fe2O3. Thus, a high transmittance is attained in a wavelength region of around 600 nm and higher.
Thus, conventionally it has been attempted to improve the glass sheets for photoelectric conversion devices by restricting the total amount of iron oxide or by controlling the oxidation/reduction state (i.e. the Fe2+/Fe3+ ratio) of the iron oxide. These glass sheets ordinarily are manufactured by the so-called float process, in which the molten glass are poured onto molten tin kept in a tin float bath, and formed into a glass sheet on that molten tin. However, according to an analysis performed by the inventors, the transmittance of soda-lime glass manufactured by the float process remains at a value that is lower than what theoretically could be accomplished with the glass composition.
Other processes for manufacturing glass sheets besides the float process are known, such as the roll-out process, the down-draw process or glass shaping on a thin layer of water vapor (see for example JP H9-295819A). However, considering the required size of the glass sheet and the manufacturing costs, the float process is even today still the most advantageous manufacturing process not only for main applications in building and vehicle glass sheets etc., but also in photoelectric conversion devices.